Field
This invention relates to electromagnetic radiation (EMR) therapy and more particularly to applicators and systems for applying electromagnetic energy to a treatment site in a living body to heat tissue needing treatment at the treatment site. The invention is useful particularly for treatments in the nature of microwave coagulation or ablation. A limitation found in current microwave antennas that are used for microwave coagulation and ablation is that the energy distribution pattern generally extends back along the applicator from the proximal end of the desired coagulation and ablation zone around the microwave energy radiating portion of the applicator toward the proximal end of the applicator. This undesirably extends the coagulation and ablation zone along the applicator toward the proximal end of the applicator beyond the tissue desired to be coagulated or ablated. This results in an elliptical or tear drop heating pattern shape where the desired shape for the energy distribution pattern and the coagulation and ablation zone is generally more spherical.
State of the Art
The use of electromagnetic (EM) energy to heat tissue for the treatment of disease is known. In using microwave energy for tissue heating, an applicator having a microwave radiating antenna is positioned with respect to the tissue to be treated (heated) so that microwave energy radiated from the antenna penetrates and heats the tissue. Many microwave applicators are known in the art. Death, or necrosis, of living tissue cells occurs at temperatures elevated above a normal cell temperature for a sufficient period of time. The sufficient period of time is generally dependent upon the temperature to which the cells are heated. Above a threshold temperature of about 41.5 degrees C., substantial thermal damage occurs in most malignant cells. At temperatures above about 45 degrees C. thermal damage occurs to most normal cells. During treatment, it is desirable to produce an elevated temperature within the targeted tissue for a time period sufficient to cause the desired cell damage, while keeping nearby healthy tissue at a safe lower temperature. For this reason, when treatment involving tissue heating is used, it is important to assure both adequate tumor heating throughout the tumor to the tumor margin and reduced temperatures in the critical normal tissue.
Heating therapy is sometimes combined with other treatments, such as surgery, ionizing radiation, and chemotherapy. For example, when heating is combined with radiation, it is desirable to maintain the temperature within the diseased tissue within the range of about 42 to 45 degrees C. Higher temperatures are usually undesirable when a combined treatment modality is used because higher temperatures can lead to microvessal collapse causing resistance to radiation therapy and decrease the amount of systemic chemotherapy from reaching the tumor if it has vascular damage. Lower temperatures are undesirable because they can fail to provide adequate therapeutic effect. Therefore, it is important to control the temperature within the desired range for multi-modality treatments and not allow heating of the tissue in the tumor or around the tumor to above 45 degrees C. if such tissue damage from other treatments may be compromised. Treatment within this controlled temperature range is usually referred to as hyperthermia.
Forms of thermal therapy that kill the tissue with heating alone are generally referred to as coagulation or ablation. To adequately eradicate a cancerous tumor with only the application of heat, it is necessary to ensure adequate heating is accomplished throughout the entire tumor. In cases of a malignant tumor, if viable tumor cells are left behind, the tumor can rapidly grow back leaving the patient with the original problem. In what is generally referred to as microwave coagulation or microwave ablation, the diseased tissue is heated to at least about 55 degrees C., and typically above about 60 degrees C., for exposure times sufficient to kill the cells, typically for greater than about one minute. With microwave coagulation and ablation treatments, there is a volume reduction of temperature that ranges from the high temperature in the treated tissue to the normal tissue temperature of 37 degrees C. outside the treated tissue. The outer margin of the overall heat distribution in the treated tissue volume may then result in damage to normal tissue if such normal tissue is overheated. Therefore, for prolonged coagulation or ablation treatments where the coagulation or ablation volume is maintained at very high temperatures, there is a high risk of damage to surrounding normal tissues. For proper treatment of targeted cancerous tumor volumes or other tissue volumes to be treated, it becomes very important to properly deliver the correct thermal distribution over a sufficient time period to eradicate the tumor tissue while minimizing damage to critical surrounding normal tissue. Fortunately, there are tumor locations that reside in normal tissue that can be destroyed by the heating in limited areas without affecting the health of the patient, such as liver tissue. In such situations the coagulation can be applied in an aggressive way to include a margin of safety in destruction of limited surrounding normal tissues to assure that all of the cancerous tumor is destroyed.
The process of heating very rapidly to high temperatures that is common in coagulation and ablation treatments may utilize a rather short exposure time. In doing so, the resulting temperature distribution becomes primarily a result of the power absorption distribution within the tissue. However, if such treatments continue for multiple minutes, the blood flow and thermal conduction of the tumor and surrounding tissues will modify the temperature distribution to result in a less predictable heat distribution because the changes occurring in blood flow in such a heated region may not be predictable. Thus, it is important to optimize the uniformity of the tissue heating power that is absorbed to lead to a more predictable temperature distribution that better corresponds with the treatment prescription. Therefore, pretreatment planning practices prior to and possibly during treatment for calculating the power and temperature distribution resulting from the parameters of power and relative phase of the power applied to the tissue can be important for not only coagulation and ablation, but also hyperthermia. As higher temperatures are used during treatment, it may increase patient discomfort and pain, so it can be helpful to avoid excessive temperatures to reduce the need of patient sedation.
Invasive microwave energy applicators can be inserted into living body tissue to place the source of heating into or adjacent to a diseased tissue area. Invasive applicators help to overcome some difficulties that surface applicators experience when the target tissue region is located below the skin (e.g., the prostrate). Invasive applicators must be properly placed to localize the heating to the vicinity of the desired treatment area. Even when properly placed, however, it has been difficult to ensure that adequate heat is developed in the diseased tissue without overheating surrounding healthy tissue. Further, with applicators operating at higher power levels to produce the needed higher temperatures for coagulation and ablation, there is a tendency for the coaxial cable in the portion of the applicator leading from outside the body to the location of the radiating antenna in the applicator to heat to undesirably high temperatures which can cause thermal damage to the normal tissue through which the applicator passes to reach the diseased tissue to be treated. Therefore, various ways of cooling the applicator have been used in the prior art.
In the use of radiofrequency (RF) electrodes in the prior art, a primary electrode is invasively inserted into the body while a secondary electrode is either invasively inserted into the body such as in a bipolar RF applicator or separately at another location or is placed on the skin outside the body so that radio frequency heating currents pass from the primary electrode to the secondary electrode. In this process, the tissue around the primary electrode has the tendency to dry out and char. When the tissue around the primary electrode that needs to pass the RF heating currents to the secondary electrode dries, the drying of the tissue around the electrode provides high electrical resistance to impede the flow of heating current. This drying of the tissue then limits the flow of current from the primary electrode out into the tissue beyond the dry or charred tissue thereby limiting the tissue coagulation zone to that which is achieved prior to the formation of this dry or charred tissue around the electrode. A technique has been developed to infuse fluid, such as saline solution, from the tip of such primary RF electrode to permit the tissue fluids to be replaced as they are being heated in that region, thereby reducing the resistance to current flow by maintaining the wetness and the electrical conductivity of the tissue near the electrode. This allows continued desired flow of current from the primary electrode to produce the desired tissue heating and coagulation. Examples of this are shown in U.S. Pat. Nos. 6,066,134, 6,112,123, and 6,131,577. It is also known in the art that fluids, such as saline solution, can be used with microwave antennas where the infusion of the fluids, such as saline, is introduced into the primary heating region to aid in the maintenance of the radiation impedance of the antenna by replacing the fluids in the tissue in the area of primary radiation and heating. Examples of this are shown in published patent applications 2006/0122593 and 2009/0248006. Other methods are also utilized to improve the impedance matching during microwave coagulation of tissue such as tuning changes by frequency change (U.S. Pat. No. 7,594,913) and variation of the exposed radiating antenna tip (published application 2009/0131926).
It is also known that with microwave energy applicators, the radiated energy forms a tear-drop type heating pattern with a tail extending back along the applicator shaft from the desired tissue heating zone into the normal tissue along the applicator insertion path toward the proximal end of the applicator. In some instances, the extent of the tail changes during treatment and with the positioning of the applicator, such as with the insertion depth of the applicator. The forming of this heated tail and the alteration of the coagulation zone as a function of variable insertion depths is not a desirable result as the coagulation zone should be consistent at various reasonable insertion depths, and the heating zone produced by the applicator is desirably a more spherical shape to produce a more spherical ablation zone.
While many microwave applicators are known in the art for applying microwave energy to tissue to provide heating to the tissue, there is a need for better applicators that are easy to use, have more consistent and predictable heating and coagulation patterns, and particularly that are able to limit the length of the energy absorption pattern and the heating zone to limit the elongation along the applicator proximally from the desired heating zone to therefore provide, when used for coagulation or ablation, a more spherical coagulation or ablation shape.